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G.B. Appetecchi et al. / Electrochemistry Communications 63 (2016) 26–29
29
conduction was also reported in Fig. 2B as a function of 103 (T − T0)−1
according to the equation:
planned procedure route. The incorporation of sulfur in the cation
alkyl side chain was found to prevent the crystallization of PYR1(2S1)TFSI
even in the presence of lithium salt, resulting in appreciable ion trans-
port (above 10−5 S cm−1) at low temperatures (−10 °C). Cyclic
voltammetries have clearly indicated that Li+ cations exhibit highly
reversible stripping/plating process and fast mobility at the interface
with lithium metal electrode. This ionic liquid is expected to be ad-
dressed as safer electrolyte component for sulfur-based battery systems
and solvent for extraction process of sulfur compounds.
ꢀ
ꢁ
B
σ ¼ AT−1=2 exp
−
ð1Þ
T−T0
where A and B are parameters correlated to the charge carrier number
and the activation energy, respectively, whereas T0 represents the
ideal glass transition temperature. The Tg value, experimentally deter-
mined by DSC measurements, was used in the place of T0, aiming to
find correlations among different experimental data. The VTF diagrams
follow a linear behavior within the 6 b 103/(T − Tg) b 19 range,
i.e., corresponding to the 0 °C/90 °C temperature interval. Below 0 °C,
the VTF plots were seen to progressively deviate from linear trend
(data not reported). The PYR1(2S1)TFSI–LiTFSI mixture (full squares) dis-
plays a higher slope with respect to the pure IL, suggesting that the ad-
dition of lithium salt does result in increasing activation energy for the
ion transport mechanism, accordingly to the lower conductivity ob-
served especially below 50 °C. Once more, this is likely due to the vis-
cous drag increase resulting from the incorporation of LiTFSI into the
ionic liquid. The VTF behavior of PYR1(2S1)TFSI (open squares) was com-
pared with that of PYR1(2O1)TFSI (open triangles) [5]. Despite superior
conductivity values, the latter ionic liquid material shows a (linear)
VTF trend with higher slope with respect to PYR1(2S1)TFSI, witnessing
for larger activation energy for the ion transport mechanism. Therefore,
the ion mobility in PYR1(2S1)TFSI seems to be slowed by higher viscous
drag but, at the same time, promoted by lower activation energy values.
Additional work, however, is required to better clarify this behavior.
A suitable electrolyte for Li/S battery systems requires high revers-
ibility of the lithium stripping/plating process in combination with
high mobility for the Li+ cation. These issues were preliminary investi-
gated by cyclic voltammetries performed on Li/PYR1(2S1)TFSI–LiTFSI/SS
asymmetric cells and the results are reported in Fig. 2C. A broad cathodic
feature, leveling 20 μA cm−2, is observed around 0.4 V (vs. Li/Li+) dur-
ing the first cycle, mainly ascribable to reduction of impurities within
the IL electrolyte, which fully disappears in the following cycles, howev-
er, allowing deposition of lithium metal (onto the working electrode).
The following cycles, characterized by well-defined profiles, show that
the lithium stripping/plating process can be conducted with very good
reversibility, i.e., both the cathodic (plating) and the anodic (stripping)
features were practically overlapped during repetitive CV tests,
supporting for good lithium deposition/dissolution efficiency. The latter
issue, highlighting good compatibility of the PYR1(2S1)TFSI–LiTFSI, sug-
gests that the presence of sulfur into the cation alkyl side chain is not
detrimental for the stability at the interface with lithium metal anode.
In addition, taking into account the relatively high scan rate
(5 mV s−1), appreciable mobility of the Li+ cation at Li/IL electrolyte in-
terface (despite the not extremely high ion conduction of PYR1(2S1)TFSI–
LiTFSI) is observed. A similar appealing behavior, not easily detectable in
ionic liquids [8], is worthy to be further investigated.
Conflict of interest
The authors declare that there are no conflict of interest.
Acknowledgments
Dr. M. Carewska of ENEA is kindly acknowledged for their assistance
in the DSC measurements.
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4. Conclusions
A novel ionic liquid never reported in literature until now,
PYR1(2S1)TFSI, formed by the N-methyl pyrrolidinium cation with a
sulfur-containing alkyl side chain and the bis(trifluorometh-
anesulfonyl)imide anion, was properly designed through suitably